Improve WIN32 port of fstat() to detect more file types

 Improve WIN32 port of fstat() to detect more file types
 
 The current implementation of _pgfstat64() is ineffective in detecting a
 terminal handle or an anonymous named pipe.  This commit improves our
 port of fstat() to detect more efficiently such cases by relying on
 GetFileType(), and returning more correct data when the type found is
 either a FILE_TYPE_PIPE (_S_IFIFO) or a FILE_TYPE_CHAR (_S_IFCHR).
 
 This is part of a more global fix to address failures when feeding the
 output generated by pg_dump to pg_restore through a pipe, for example,
 but not all of it.   We are also going to need to do something about
 fseek() and ftello() which are not reliable on WIN32 for the same cases
 where fstat() was incorrect.  Fixing fstat() is independent of the rest,
 though, which is why both fixes are handled separately, and this is the
 first part of it.
 
 Reported-by: Daniel Watzinger
 Author: Daniel Watzinger, Juan José Santamaría Flecha
 Discussion: https://postgr.es/m/b1448cd7-871e-20e3-8398-895e2d1d3bf9@gmail.com
 Backpatch-through: 14